1. Field of the Invention
The present invention relates to a power supply control circuit. In particular, the present invention relates to a power supply control circuit having a power supply reverse connection protection function for protecting an output transistor against overpower when a power supply is reversely connected (hereinafter, referred to as “power supply reverse connection protection function”), and also having an overvoltage protection function for protecting the output transistor against overvoltage that may be applied from a power supply line or an output terminal when the power supply is normally connected (hereinafter, referred to as “overvoltage protection function”).
2. Description of Related Art
Japanese Unexamined Patent Application No. 2007-019812 (hereinafter, referred to as “Patent Document 1”), for example, discloses a power supply control circuit having a power supply reverse connection protection function. The power supply control circuit disclosed in Patent Document 1 is described with reference to FIG. 1.
A power supply control circuit 200 includes an output transistor 109, a load 112, and a driver circuit 113. The output transistor 109 is connected between a power supply line 101 and an output terminal 106. The power supply line 101 is electrically connected to a positive terminal of a battery power supply. The load 112 is connected between the output terminal 106 and a power supply line 102. The driver circuit 113 controls a conductive/non-conductive state of the output transistor 109 through a gate resistor 107.
To deal with the case where a power supply is reversely connected, a transistor 114 is connected between a power supply line 103 and an output transistor 116. The power supply line 103 is electrically connected to a negative terminal of the battery power supply. Operations thereof are described in Patent Document 1. When the power supply is reversely connected, the power supply lines 101 and 103 are electrically connected with the negative terminal and the positive terminal of the battery power supply, respectively. An electric charge is applied to the output transistor 109 through a parasitic diode 115 of the transistor 114, with the result that the output transistor 109 is rendered conductive and protected.
Meanwhile, Japanese Unexamined Patent Application Publication No. 2007-028747 (hereinafter, referred to as “Patent Document 2”), for example, discloses a power supply control circuit having an overvoltage protection function. The structure of the power supply control circuit is shown in FIG. 2, and components identical with those shown in FIG. 1 are denoted by the same reference numerals. A power supply control circuit 100 additionally includes a gate charge discharging transistor 108, a switch transistor 110, and a dynamic clamp circuit 111.
The overvoltage protection function is executed by the switch transistor 110 and the dynamic clamp circuit 111, and is obtained by applying a ground potential 130 to a gate of the switch transistor 110 as a reference voltage. Note that the ground potential 130 is electrically connected with the negative terminal of the battery power supply. Operations thereof are described in detail in Patent Document 2, so a description thereof is herein omitted.
Note that the power supply line 102 serving as a second power supply connected to one end of the load 112 as shown in FIGS. 1 and 2 is connected to the negative terminal of the battery power supply.
In view of the above, there is a demand for a circuit having both the overvoltage protection function as shown in FIG. 1 and the power supply reverse connection protection function as shown in FIG. 2. In this case, the switch transistor 110 and the clamp circuit 111 may be provided, as shown in FIG. 2, between a gate of the transistor 109 shown in FIG. 1 and the power supply line 10. The present inventor has found that the structure, however, has a problem in that the protection function is inhibited in the state where the power supply is reversely connected.
In other words, the reference voltage applied to the gate of the switch transistor 110 is connected with a ground voltage during normal operation. For this reason, an electrical path is generally formed between the reference voltage applied to the switch transistor 110, and the power supply line 103. Accordingly, even in the case where the power supply is reversely connected, once the electrical path is activated, the power supply line 103 is set at a potential higher than that of the power supply line 101. Thus, the switch transistor 110 is rendered conductive and the diode 111 is biased in a forward direction, so the electric charge supplied to the gate of the output transistor 109 through the parasitic diode 115 of the transistor 114 is drawn into the switch transistor 110 and the diode 111. As a result, the output transistor 109 is not rendered conductive.